Resin composition and power cable

Abstract

The resin composition comprises: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted onto the polyolefin; and an inorganic filler comprising magnesium oxide, which is surface-treated with a silane coupling agent, and the residual rate R of the inorganic filler in the frozen fracture surface is 50% or more. The residual rate R is calculated by the following formula (1). R = (Df / Dc) × 100……(1). Df is the density of the inorganic filler remaining in the fracture surface of a sheet of a resin composition comprising a base resin, a modified polyolefin, and an inorganic filler, and the sheet is immersed in liquid nitrogen for 1 hour and then fractured. Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet of the resin composition is not frozen but cut using a focused ion beam at 24°C.

Description

Technical Field

[0001] The present disclosure relates to a resin composition and a power cable.

[0002] This application claims priority based on Japanese patent applications No. 2021-13274 filed on January 29, 2021 and No. 2021-13276 filed on the same day, and incorporates all the contents described in those Japanese applications. Background Art

[0003] In recent years, solid-insulated DC power cables (hereinafter referred to as "DC power cables") have been developed for DC power transmission applications. In DC power cables, when high voltage is applied, space charge is generated within the insulation layer, potentially degrading the insulation's DC characteristics (volume resistivity, DC breakdown electric field strength, and space charge characteristics).

[0004] Therefore, in order to suppress the accumulation of space charge in the insulation layer of a DC power cable, a polar inorganic filler such as carbon black or magnesium oxide (MgO) is sometimes added to the resin composition constituting the insulation layer (for example, Patent Document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 11-16421 Summary of the Invention

[0008] According to one embodiment of the present disclosure, a resin composition is provided, comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted onto the polyolefin; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein a residual ratio R of the inorganic filler in a frozen fracture surface is 50% or more, wherein the residual ratio R is obtained by the following formula (1): R=(Df / Dc)×100……(1), Df is the density of the inorganic filler remaining in the fracture surface of a sheet of the resin composition comprising the base resin, the modified polyolefin, and the inorganic filler, when the sheet is immersed in liquid nitrogen for 1 hour and then fractured, and Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet of the resin composition is not frozen but cut using a focused ion beam at 24°C.

[0009] According to another embodiment of the present disclosure, there is provided a power cable comprising: a conductor; and an insulating layer provided to cover the outer periphery of the conductor, the insulating layer being composed of a resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted thereto; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein a residual ratio R of the inorganic filler in a frozen fracture surface is 50% or more, wherein the residual ratio R is obtained by the following formula (1): R=(Df / Dc)×100……(1), Df is the density of the inorganic filler remaining in the fracture surface of a sheet forming the insulating layer when the sheet is immersed in liquid nitrogen for 1 hour and then fractured, and Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet is cut at 24°C using a focused ion beam without being frozen.

[0010] According to another embodiment of the present disclosure, there is provided a power cable comprising: a conductor; and an insulating layer provided to cover the outer periphery of the conductor, wherein the insulating layer is composed of a resin composition, the resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted thereto; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein the content N of the inorganic filler in the resin composition is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin, and the following formula (2) is satisfied: V < 2.5N + 67.5……(2), wherein V is the brightness in the HSV color space with the unit being % when a sheet having the insulating layer having a thickness of 2 mm is formed, the sheet is bent at 24°C, and the cross section of the bent portion of the sheet is observed, V is set to 50% when a cross section is observed when a sheet having an insulating layer not containing the modified polyolefin and the inorganic filler is bent under the same conditions, and V is set to 80% when a sheet having an insulating layer not containing the modified polyolefin and containing 5 parts by mass of an unsurface-treated inorganic filler is bent under the same conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 1 is a schematic cross-sectional view perpendicular to the axial direction of a DC power cable according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0012] [Problems to be Solved by the Present Disclosure]

[0013] An object of the present disclosure is to provide a technology capable of suppressing degradation of DC characteristics of a DC power cable caused by temperature changes.

[0014] [Effects of the Present Disclosure]

[0015] According to the present disclosure, degradation of DC characteristics of a DC power cable caused by temperature changes can be suppressed.

[0016] [Description of Embodiments of the Present Disclosure]

[0017] <Insights Obtained by the Inventors>

[0018] First, the findings obtained by the inventors will be briefly described.

[0019] (i) Knowledge on Adhesion of Inorganic Fillers

[0020] By dispersing the aforementioned inorganic fillers within the insulating layer, space charge generated within the insulating layer can be trapped within each inorganic filler. This suppresses the accumulation of localized space charge within the insulating layer, improving the DC characteristics of the DC power cable. Therefore, to achieve the desired DC characteristics of a DC power cable, it is important to appropriately incorporate the inorganic fillers within the insulating layer.

[0021] Here, the DC power cable may be installed in an environment with a large temperature fluctuation range, for example. In conventional DC power cables, the DC characteristics may be degraded (decreased, worsened) in an environment with a large temperature fluctuation range.

[0022] The inventors conducted extensive research on this topic and discovered that the degradation of DC characteristics of DC power cables with temperature changes depends on the adhesion of inorganic fillers added to the insulation layer of the DC power cables. This is believed to be based on the following mechanism.

[0023] When the temperature of the installation environment fluctuates, the insulation layer in a DC power cable expands and contracts. This thermal expansion and contraction exerts thermal stress between the base resin and the inorganic filler that make up the insulation layer. Larger temperature fluctuations increase the thermal stress. Furthermore, when the temperature fluctuations occur over a long period of time, the effects of thermal stress accumulate in the insulation layer.

[0024] If the adhesion between the base resin and the inorganic filler is low, or if this adhesion decreases over time, thermal stress generated by the thermal expansion and contraction of the insulation layer, or the accumulation of this thermal stress, may cause the inorganic filler to separate from the base resin, creating gaps between the base resin and the inorganic filler. The formation of gaps between the base resin and the inorganic filler can locally reduce the insulation properties of the insulation layer, and the inorganic filler's ability to capture space charge can also decrease. Consequently, temperature fluctuations can degrade the DC characteristics of the DC power cable.

[0025] (ii) Knowledge on the surface treatment process of inorganic fillers

[0026] Through further in-depth research, the inventors discovered that, in the manufacturing process of a DC power cable, the conditions in the process of surface treating an inorganic filler with a silane coupling agent affect the adhesion of the inorganic filler.

[0027] During the surface treatment process, for example, if the amount of moisture in the atmosphere during surface treatment of the inorganic filler is high (the silane coupling agent may be excessively hydrolyzed), the silane coupling agents may bond with each other in the atmosphere. When the silane coupling agents bond with each other in the atmosphere, the density of the silane coupling agents in the atmosphere decreases, and the coverage of the inorganic filler by the silane coupling agents decreases. Consequently, the adhesion between the inorganic filler and the base resin may decrease.

[0028] In addition, during the surface treatment process, for example, when the amount of moisture in the atmosphere during the surface treatment of the inorganic filler is high, the inorganic filler will absorb moisture. When the inorganic filler absorbs moisture during the surface treatment process, the inorganic fillers may condense with each other. In addition, even if the initial moisture content of the inorganic filler before the surface treatment process is high, the inorganic fillers may condense with each other. In particular, when the surface treatment process is dry, it becomes impossible to fully stir the particles of the inorganic filler, and the inorganic fillers are more likely to condense with each other. When the inorganic fillers condense with each other, it becomes impossible to fully implement the surface treatment performed by the silane coupling agent on the contact portion between the condensed inorganic fillers, and the coverage of the inorganic filler by the silane coupling agent will decrease. For this reason, the adhesion of the inorganic filler to the base resin may also decrease.

[0029] The present disclosure is based on the above-mentioned findings (i) and (ii) found by the inventors.

[0030] <Embodiments of the present disclosure>

[0031] Next, embodiments of the present disclosure will be described by way of examples.

[0032] [1] A resin composition according to one embodiment of the present disclosure comprises: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted onto the polyolefin; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein the residual rate R of the inorganic filler in a frozen fracture surface is 50% or more, wherein the residual rate R is obtained by the following formula (1): R = (Df / Dc) × 100 ... (1), Df is the density of the inorganic filler remaining in the fracture surface of a sheet of a resin composition comprising the base resin, the modified polyolefin, and the inorganic filler, when the sheet is immersed in liquid nitrogen for 1 hour and then fractured, and Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet of the resin composition is cut at 24°C using a focused ion beam without being frozen.

[0033] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be suppressed.

[0034] [2] Another embodiment of the present disclosure relates to a power cable comprising: a conductor; and an insulating layer configured to cover the outer periphery of the conductor, wherein the insulating layer is composed of a resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted thereto; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein the residual rate R of the inorganic filler in a frozen fracture surface is 50% or more, wherein the residual rate R is obtained by the following formula (1): R = (Df / Dc) × 100 ... (1), Df is the density of the inorganic filler remaining in the fracture surface of a sheet forming the insulating layer when the sheet is immersed in liquid nitrogen for 1 hour and then fractured, and Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet is cut at 24°C using a focused ion beam without being frozen.

[0035] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be suppressed.

[0036] [3] In the power cable described in [2] above, after a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 1 hour, wherein in the heat cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0037] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be suppressed.

[0038] [4] In the power cable described in [2] or [3] above, the inorganic filler is surface-treated with a material other than a silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon atom.

[0039] According to this configuration, the adhesion between the inorganic filler and the base resin can be significantly improved.

[0040] [5] In the power cable described in [4] above, the inorganic filler is selected from the group consisting of vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldieth ... The surface of the composite material is treated with at least one of oxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylene)propylamine, dimethyldimethoxysilane, methoxytrimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, and trimethylchlorosilane.

[0041] According to this configuration, the adhesion between the inorganic filler and the base resin can be significantly improved.

[0042] [6] In the power cable described in any one of [2] to [5] above, the inorganic filler is surface-treated with a material having a vinyl group at its terminal as the silane coupling agent.

[0043] According to this configuration, the adhesion between the inorganic filler and the base resin can be significantly improved.

[0044] [7] In the power cable according to any one of [4] to [6] above, the residual ratio R is 60% or more.

[0045] According to this configuration, degradation of the DC characteristics of the power cable due to temperature changes can be significantly suppressed.

[0046] [8] In the power cable described in any one of [4] to [7] above, after a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 2 hours, wherein in the heat cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0047] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be significantly suppressed.

[0048] [9] Another embodiment of the present disclosure provides a power cable comprising: a conductor; and an insulating layer provided to cover the outer periphery of the conductor, wherein the insulating layer is composed of a resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted thereto; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein the content N of the inorganic filler in the resin composition is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin, and the following formula (2) is satisfied: V < 2.5N + 67.5… …(2), wherein V is the lightness in the HSV color space with the unit being % when a sheet having the insulating layer having a thickness of 2 mm is formed, the sheet is bent at 24°C, and the cross section of the bent portion of the sheet is observed, V is set to 50% when a cross section is observed when a sheet having an insulating layer not containing the modified polyolefin and the inorganic filler is bent under the same conditions, and V is set to 80% when a sheet having an insulating layer not containing the modified polyolefin and containing 5 parts by mass of an unsurface-treated inorganic filler is bent under the same conditions.

[0049] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be suppressed.

[0050]

[10] In the power cable described in [9] above, after a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 1 hour, wherein in the heat cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0051] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be suppressed.

[0052]

[11] In the power cable described in [9] or

[10] above, the inorganic filler is surface-treated with a material other than a silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon atom.

[0053] According to this configuration, the adhesion between the inorganic filler and the base resin can be significantly improved.

[0054]

[12] In the power cable described in

[11] above, the inorganic filler is selected from the group consisting of vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldieth ... The surface of the composite material is treated with at least one of acyloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylene)propylamine, dimethyldimethoxysilane, methoxytrimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, and trimethylchlorosilane.

[0055] According to this configuration, the adhesion between the inorganic filler and the base resin can be significantly improved.

[0056]

[13] In the power cable described in any one of [9] to

[12] above, the inorganic filler is surface-treated with a material having a vinyl group at its terminal as the silane coupling agent.

[0057] According to this configuration, the adhesion between the inorganic filler and the base resin can be significantly improved.

[0058]

[14] In the power cable described in any one of

[11] to

[13] above, the following formula (3) is satisfied: V<5N+55……(3).

[0059] According to this configuration, degradation of the DC characteristics of the power cable due to temperature changes can be significantly suppressed.

[0060]

[15] In the power cable described in any one of

[11] to

[14] above, after a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 2 hours, wherein in the heat cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0061] According to this configuration, degradation of the DC characteristics of the DC power cable due to temperature changes can be significantly suppressed.

[0062] [Details of the embodiments of the present disclosure]

[0063] Next, an embodiment of the present disclosure will be described below with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to these examples, but is shown by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0064] <One embodiment of the present disclosure>

[0065] (1) Resin composition

[0066] The resin composition of the present embodiment is a material constituting the insulating layer 130 of the DC power cable 10 described later, and includes, for example, a base resin, a modified polyolefin, an inorganic filler, a crosslinking agent, and other additives.

[0067] (Base resin)

[0068] The base resin (base polymer) refers to the resin component that constitutes the main component of the resin composition. The base resin of the present embodiment includes, for example, polyolefin. As the polyolefin constituting the base resin, for example, polyethylene, polypropylene, ethylene-α-olefin copolymer, a thermoplastic elastomer formed by dispersing or copolymerizing ethylene-propylene rubber in polypropylene, etc. can be listed. Among them, polyethylene is preferred. It should be noted that two or more of them can also be used in combination.

[0069] Examples of the polyethylene constituting the base resin include low-density polyethylene (LDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE). These polyethylenes may be linear or branched.

[0070] (Modified polyolefin)

[0071] The modified polyolefin contained in the resin composition of this embodiment has, for example, a polar group grafted thereto. By including such a modified polyolefin, the compatibility (adhesion) between the polar inorganic filler and the base resin can be improved, thereby improving the dispersibility of the inorganic filler.

[0072] The polyolefin constituting the modified polyolefin is, for example, the same as the polyolefin contained in the base resin.

[0073] The modified polyolefin is composed of, for example, an acid-modified polyolefin. The polar group grafted into the acid-modified polyolefin is, for example, at least one of maleic anhydride and phthalic anhydride.

[0074] The modification ratio (copolymerization ratio) of the polar groups in the modified polyolefin relative to the polyolefin is not particularly limited, but is, for example, 0.1% or more and 0.5% or less. By setting the modification ratio of the polar groups relative to the polyolefin to 0.1% or more, the affinity between the modified polyolefin and the inorganic filler can be improved, thereby enhancing the dispersibility of the inorganic filler. On the other hand, by setting the modification ratio of the polar groups relative to the polyolefin to 0.5% or less, separation of the modified polyolefin from the base resin in the resin composition can be suppressed.

[0075] The content of the modified polyolefin in the resin composition is not particularly limited; for example, it is 0.5 parts by mass or more and 10 parts by mass or less relative to 100 parts by mass of the base resin. By setting the modified polyolefin content to 0.5 parts by mass or more, the dispersibility of the inorganic filler can be sufficiently improved. On the other hand, by setting the modified polyolefin content to 10 parts by mass or less, scorch (premature crosslinking) can be suppressed during extrusion molding of the insulation layer 130.

[0076] (Inorganic filler)

[0077] The inorganic filler acts by capturing space charge in the insulating layer 130, thereby suppressing localized accumulation of space charge in the insulating layer 130. This improves the DC characteristics of the insulating layer 130. It should be noted that the "DC characteristics of the insulating layer 130" or "DC characteristics of the DC power cable 10" referred to herein refer to the volume resistivity, DC breakdown electric field strength, and space charge characteristics of the insulating layer 130.

[0078] The inorganic filler includes, for example, magnesium oxide (MgO). Methods for forming MgO as an inorganic filler include, for example, a vapor phase method in which Mg vapor is brought into contact with oxygen, or a seawater method using a seawater raw material. The inorganic filler in this embodiment may be formed by either the vapor phase method or the seawater method.

[0079] In this embodiment, the content of the inorganic filler in the resin composition (hereinafter referred to as N) is not particularly limited, and is, for example, 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin. By setting the content of the inorganic filler to 0.1 parts by mass or more, the inorganic filler can fully capture space charge. On the other hand, by setting the content of the inorganic filler to 5 parts by mass or less, the moldability of the resin composition can be improved, and the dispersibility of the inorganic filler in the insulating layer 130 can be improved.

[0080] In the present embodiment, the volume average particle size (MV: Mean Volume Diameter) (X described below) of the inorganic filler is not particularly limited, but is, for example, 1 μm or less, and preferably 700 nm or less.

[0081] It should be noted that when the particle size is set to d i , set the volume of the particle to V i When , the "volume average particle size (MV)" referred to herein is calculated by the following formula.

[0082] MV=Σ(V i d i ) / ΣV i

[0083] In addition, the volume average particle diameter was measured using a dynamic light scattering particle size / particle size distribution measuring device.

[0084] By setting the volume average particle size of the inorganic filler to 1 μm or less, the inorganic filler can be uniformly dispersed in the insulating layer 130. This allows the inorganic filler to stably improve the DC characteristics. Furthermore, by setting the volume average particle size of the inorganic filler to 700 nm or less, the inorganic filler can be more easily uniformly dispersed in the insulating layer 130. This allows the inorganic filler to more stably improve the DC characteristics.

[0085] In addition, the lower limit of the volume average particle size of the inorganic filler is not particularly limited.

[0086] However, from the viewpoint of stably forming the inorganic filler, the volume average particle size of the inorganic filler is, for example, 1 nm or more, or preferably 5 nm or more.

[0087] In this embodiment, at least a portion of the inorganic filler is surface-treated with a silane coupling agent. In other words, at least a portion of the surface of the inorganic filler has, for example, a silyl group containing a predetermined organic substituent. This improves the adhesion of the interface between the inorganic filler and the base resin, thereby enhancing the mechanical and low-temperature properties of the insulating layer 130.

[0088] In this embodiment, as described below, by reducing the moisture content in the atmosphere during the inorganic filler surface treatment process, the decrease in the coverage of the inorganic filler by the silane coupling agent is suppressed. In other words, the decrease in the coverage of the inorganic filler surface by silyl groups is suppressed. By reducing the moisture content during the surface treatment, this effect can be achieved to a certain extent regardless of the type of silane coupling agent used for the surface treatment. As a result, the adhesion between the inorganic filler and the base resin can be improved.

[0089] On the other hand, in this embodiment, the inorganic filler is preferably surface-treated with a material other than a silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon (Si) atom, for example. It should be noted that silane coupling agents having three hydrolyzable groups and only one alkyl group per Si atom are hereinafter also referred to as "monoalkylsilane coupling agents."

[0090] Examples of silane coupling agents that satisfy the above-mentioned requirements include vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyl Acyloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylene)propylamine, dimethyldimethoxysilane, methoxytrimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, trimethylchlorosilane, etc. It should be noted that two or more of these may be used in combination.

[0091] By surface-treating at least a portion of the inorganic filler with a silane coupling agent other than a monoalkylsilane coupling agent, at least a portion of the surface of the inorganic filler has silyl groups other than monoalkylsilyl groups. It should be noted that a monoalkylsilyl group refers to a silyl group in which the organic substituents, excluding any remaining hydrolyzable groups, consist solely of independent alkyl groups. While the detailed mechanism is unknown, this configuration significantly improves the adhesion between the inorganic filler and the base resin compared to a case where the inorganic filler is surface-treated with a monoalkylsilane coupling agent.

[0092] Alternatively, in the present embodiment, the inorganic filler is preferably surface-treated using a material having a vinyl group at the end as a silane coupling agent. It should be noted that, hereinafter, the silane coupling agent having a vinyl group at the end is also referred to as a "vinyl silane coupling agent". In this way, by utilizing a vinyl silane coupling agent to surface-treat at least a portion of the inorganic filler, at least a portion of the surface of the inorganic filler has a silyl group containing a vinyl group at the end. Thus, in the cross-linking step thereafter, a specified cross-linking agent can be used to cross-link the vinyl group derived from the vinyl silane coupling agent with the base resin. As a result, the adhesion of the inorganic filler to the base resin can be significantly improved.

[0093] Note that, as long as the vinylsilane coupling agent has a vinyl group at the terminal, even materials other than the above-mentioned vinyltrimethoxysilane and vinyltriethoxysilane can achieve the above-mentioned effect of improving the adhesion of the inorganic filler.

[0094] Specifically, examples of the vinylsilane coupling agent other than the materials listed above include at least one of allyltrimethoxysilane, 7-octenyltrimethoxysilane, and vinyltrichlorosilane.

[0095] [Residual density of inorganic filler in frozen fracture surface]

[0096] In this embodiment, by performing the surface treatment step described later using the above-described silane coupling agent, the adhesion of the inorganic filler added to the insulating layer 130 to the base resin is stably improved and maintained over time.

[0097] However, it is difficult to directly measure the adhesion between each inorganic filler and the base resin.

[0098] Therefore, as a method for evaluating the adhesion between inorganic fillers and base resins, the present inventors froze a sheet of insulating layer 130 and then fractured it. They then measured the density of the inorganic filler remaining on the fractured surface of the sheet. This measurement prevents inorganic fillers with low adhesion from scattering during fracture after freezing, preventing them from remaining on the fractured surface. By measuring this residual density, the adhesion between each inorganic filler and base resin can be determined. It should be noted that this density of inorganic filler is hereinafter referred to as the "residual density of inorganic filler in the frozen fractured surface."

[0099] As a result of these measurements, the present inventors discovered that the DC characteristics of the DC power cable 10 after a predetermined thermal cycle correlate with the residual density of the inorganic filler in the freeze-fractured surface. Furthermore, the present inventors discovered that the initial DC characteristics of the DC power cable 10 also correlate with the residual density of the inorganic filler in the freeze-fractured surface.

[0100] However, the density of the inorganic filler before freeze-fracture may differ between insulating layers 130 having different inorganic filler contents or volume average particle sizes. Therefore, the residual density of the inorganic filler in the freeze-fracture surface cannot be directly compared between different insulating layers 130.

[0101] Therefore, the present inventors compared the ratio of the residual density of the inorganic filler after freeze-fracture to the density of the inorganic filler before freeze-fracture. They found that, regardless of the inorganic filler content and volume average particle size, the DC power cable 10 of this embodiment met the necessary conditions described below. It should be noted that this ratio is hereinafter referred to as the "residual ratio R of the inorganic filler in the freeze-fracture surface."

[0102] That is, in the present embodiment, the residual ratio R of the inorganic filler in the frozen fracture surface is, for example, 50% or more, or preferably 60% or more.

[0103] The residual ratio R (unit: %) is obtained from the following formula (1).

[0104] R=(Df / Dc)×100……(1)

[0105] Df is the residual density of the inorganic filler after the above-mentioned freeze fracture (the residual density in the frozen fracture surface). That is, Df is the density of the inorganic filler remaining in the fracture surface of the sheet forming the insulating layer 130 when the sheet is immersed in liquid nitrogen for 1 hour and then frozen (bent) and then fractured. In addition, Dc is equivalent to the density of the inorganic filler before the above-mentioned freeze fracture. That is, Dc is the reference density of the inorganic filler detected in the cut surface of the sheet when the sheet of the insulating layer 130 is not frozen and the sheet is cut using a focused ion beam at 24°C. It should be noted that "24°C" refers to the ambient temperature during focused ion beam processing.

[0106] [Brightness of the bent cross section of the insulation layer]

[0107] Furthermore, as a method for evaluating the adhesion between the inorganic filler and the base resin, the present inventors measured the lightness V of the bent cross-section of the insulating layer 130 in the following manner. Specifically, first, a sheet of the insulating layer 130 having a thickness of 2 mm was formed. After the sheet was formed, the sheet was bent 180° at 24°C, and the cross-section of the bent portion of the sheet was observed (photographed) using an optical microscope. At this time, a histogram analysis was performed on the obtained cross-sectional image using image analysis software, thereby determining the lightness V in the HSV (Hue, Saturation, Value) color space. It should be noted that the HSV color space mentioned here refers to a color space composed of hue H, saturation S, and lightness V.

[0108] The greater the lightness V of the bent cross-section of the insulating layer 130, the more white the bent cross-section of the insulating layer 130 appears, which corresponds to lower adhesion between the inorganic filler and the base resin. On the other hand, the smaller the lightness V of the bent cross-section of the insulating layer 130, the more suppressed the whitening of the bent cross-section of the insulating layer 130 appears, which corresponds to higher adhesion between the inorganic filler and the base resin.

[0109] The whitening of the bent cross-section of the insulating layer 130 mentioned here can be considered to be a phenomenon that occurs, for example, according to the following mechanism. When the adhesion between the inorganic filler and the base resin is low, the stress caused by bending the insulating layer 130 causes the inorganic filler to peel from the base resin, resulting in microscopic voids between the base resin and the inorganic filler. When voids form between the base resin and the inorganic filler, light is scattered by the voids in the bent cross-section of the insulating layer 130, making the cross-section appear whitened. Therefore, by measuring the brightness V of the bent cross-section of the insulating layer 130, an index value corresponding to the number of voids generated in the bent cross-section of the insulating layer 130 can be determined. As a result, the adhesion between the inorganic filler and the base resin can be evaluated.

[0110] It should be noted that, in the present embodiment, the observation conditions (for example, the brightness of the light source, etc.) when measuring the brightness V of the bent cross-section of the insulating layer 130 are set as follows. It should be noted that the unit of brightness V is %. Specifically, the brightness V when observing the cross-section formed by bending a sheet of an insulating layer containing no modified polyolefin and an inorganic filler under the same conditions is set to 50%. In addition, the brightness V when observing the cross-section formed by bending a sheet of an insulating layer containing no modified polyolefin and 5 parts by mass of an inorganic filler that has not been surface treated under the same conditions is set to 80%. It should be noted that the base resin of the resin composition used as a benchmark in the setting of brightness V is the same polyolefin as the resin composition of the present embodiment. Under such observation conditions, the brightness V of the bent cross-section of the insulating layer 130 of the present embodiment is measured.

[0111] As a result of this measurement, the present inventors discovered that the bending stress of the insulating layer 130 during bending is considered equivalent to the thermal stress during a predetermined thermal cycle. Therefore, the DC characteristics of the DC power cable 10 after the predetermined thermal cycle are correlated with the lightness V of the bent cross-section of the insulating layer 130. Furthermore, the present inventors discovered that the initial DC characteristics of the DC power cable 10 are also correlated with the lightness V of the bent cross-section of the insulating layer 130.

[0112] Specifically, in this embodiment, the following formula (2) is satisfied, and preferably the following formula (3) is satisfied.

[0113] V<2.5N+67.5……(2)

[0114] V<5N+55……(3)

[0115] As described above, in this embodiment, the necessary condition that "the residual density of the inorganic filler in the frozen fracture surface is greater than 50%, preferably greater than 60%" and the necessary condition that "the brightness V of the bent cross-section of the insulating layer 130 satisfies formula (2), preferably satisfies formula (3)" are also collectively referred to as "necessary conditions for the adhesion of the inorganic filler."

[0116] [DC Characteristics Obtained by Satisfying the Adhesion Requirements of Inorganic Fillers]

[0117] By satisfying the requirements for the adhesion of the inorganic filler as described above, the DC characteristics of the DC power cable 10 after a predetermined heat cycle can be improved, and the initial DC characteristics of the DC power cable 10 can be improved.

[0118] When the residual rate R of the inorganic filler in the frozen fracture surface is less than 50% and the brightness V of the bent cross section of the insulating layer 130 does not satisfy formula (2), the inorganic filler may be peeled off from the base resin due to the thermal stress generated during the thermal expansion and contraction of the insulating layer 130, or the accumulation of the influence of the thermal stress, resulting in a gap between the base resin and the inorganic filler. When gaps are generated between the base resin and the inorganic filler, the insulation properties of the insulating layer will be locally reduced, and the inorganic filler's ability to capture space charges will be reduced. As a result, the DC characteristics of the DC power cable 10 may be degraded (decreased, worsened) due to temperature changes. Specifically, after a prescribed thermal cycle test, when a DC electric field of 200 kV / mm is applied at a temperature of 90°C in the thickness direction of the insulating layer 130, the time until insulation breakdown of the insulating layer 130 may be less than 1 hour.

[0119] Furthermore, if the residual ratio R of the inorganic filler in the freeze-fractured surface is less than 50%, and the lightness V of the bent cross-section of the insulating layer 130 does not satisfy equation (2), the adhesion between the inorganic filler and the base resin may be low even before a temperature change occurs. In this case, the space charge generated in the insulating layer 130 cannot be fully captured in the inorganic filler, and localized space charge accumulation may occur in the insulating layer. As a result, the DC characteristics of the DC power cable 10 may deteriorate even in its initial state before a temperature change occurs.

[0120] In contrast, in this embodiment, by ensuring that the residual rate R of the inorganic filler in the frozen fracture surface is 50% or greater, or by ensuring that the lightness V of the bent cross-section of the insulating layer 130 satisfies equation (2), the inorganic filler can be prevented from peeling off from the base resin due to thermal stress generated during thermal expansion and contraction of the insulating layer 130. Furthermore, even if the temperature change period is long and the effects of thermal stress accumulate in the insulating layer 130, the inorganic filler can be prevented from peeling off from the base resin due to the accumulation of thermal stress. By suppressing the peeling of the inorganic filler in this way, the formation of voids between the base resin and the inorganic filler can be suppressed. This prevents a localized decrease in the insulation properties of the insulating layer 130 and a decrease in the inorganic filler's ability to capture space charge. Consequently, degradation of the DC characteristics of the DC power cable 10 caused by temperature changes can be suppressed. Specifically, after a predetermined heat cycle test, when a DC electric field of 200 kV / mm is applied at 90° C. in the thickness direction of insulating layer 130 , the time until insulating layer 130 undergoes dielectric breakdown can be set to 1 hour or longer.

[0121] Furthermore, by ensuring that the residual ratio R of the inorganic filler in the frozen fracture surface is 60% or greater, or that the lightness V of the bent cross-section of the insulating layer 130 satisfies equation (3), degradation of the DC characteristics of the DC power cable 10 caused by temperature changes can be significantly suppressed. Specifically, after a predetermined thermal cycle test, when a DC electric field of 200 kV / mm is applied through the thickness direction of the insulating layer 130 at a temperature of 90°C, the time until dielectric breakdown of the insulating layer 130 is reduced to 2 hours or greater.

[0122] Furthermore, in this embodiment, by ensuring that the residual ratio R of the inorganic filler in the frozen fracture surface is 50% or greater, or by ensuring that the lightness V of the bent cross-section of the insulating layer 130 satisfies equation (2), the inorganic filler maintains high adhesion to the base resin even before a temperature change occurs. This allows the space charge generated in the insulating layer 130 to be fully trapped in the inorganic filler, suppressing the accumulation of localized space charge in the insulating layer 130. Consequently, the DC characteristics of the DC power cable 10 in its initial state, before a temperature change occurs, can be improved.

[0123] (cross-linking agent)

[0124] The crosslinking agent is, for example, an organic peroxide. Examples of the organic peroxide include dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and 1,3-bis(tert-butylperoxyisopropyl)benzene. It should be noted that two or more of these can be used in combination.

[0125] (Other additives)

[0126] The resin composition may further contain, for example, an antioxidant and a lubricant.

[0127] Examples of the antioxidant include 2,2-thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 2,4-bis[(octylthio)methyl]-o-cresol, 2,4-bis(n-octylthio)-6-(4-hydroxy-3,5-di-tert-butylanilino)-1,3,5-triazine, bis[2-methyl-4-{3-n-alkyl(C12 or C14)thiopropionyloxy}-5-tert-butylphenyl]sulfide, and 4,4′-thiobis(3-methyl-6-tert-butylphenol). Two or more of these may be used in combination.

[0128] The lubricant acts in a manner that suppresses the aggregation of the inorganic filler and improves the fluidity of the resin composition during the extrusion molding of the insulating layer 130. The lubricant of this embodiment is, for example, a fatty acid metal salt or a fatty acid amide. Examples of fatty acid metal salts include magnesium stearate, zinc stearate, aluminum stearate, and magnesium montanate. In addition, examples of fatty acid amides include oleamide and stearamide. It should be noted that two or more of these may also be used in combination.

[0129] In addition, the resin composition may further contain a colorant, for example.

[0130] (2) DC power cable

[0131] Next, use Figure 1 A DC power cable according to this embodiment will be described. Figure 1 This is a cross-sectional view perpendicular to the axial direction of the DC power cable according to the present embodiment.

[0132] The DC power cable 10 of this embodiment is configured as a so-called solid-insulated DC power cable, and includes, for example, a conductor 110 , an inner semiconductive layer 120 , an insulating layer 130 , an outer semiconductive layer 140 , a shield layer 150 , and a sheath 160 .

[0133] (Conductor (conductive part))

[0134] The conductor 110 is formed by twisting together a plurality of conductor core wires (conductive core wires) made of, for example, pure copper, a copper alloy, aluminum, or an aluminum alloy.

[0135] (Inner semiconducting layer)

[0136] Internal semiconductive layer 120 is provided to cover the outer periphery of conductor 110. Furthermore, internal semiconductive layer 120 is configured to have semiconductivity and to suppress electric field concentration on the surface side of conductor 110. Internal semiconductive layer 120 may contain, for example, at least one of ethylene-ethyl acrylate copolymer, ethylene-methyl acrylate copolymer, ethylene-butyl acrylate copolymer, and ethylene-vinyl acetate copolymer, and conductive carbon black.

[0137] (Insulation layer)

[0138] Insulating layer 130 is provided to cover the outer periphery of internal semiconductive layer 120. Insulating layer 130 is composed of the resin composition of this embodiment described above.

[0139] In the present embodiment, the inorganic filler is firmly adhered to the base resin constituting the insulating layer 130 by performing a surface treatment step described later.

[0140] In this embodiment, insulating layer 130 is crosslinked by, for example, extruding and heating the resin composition of this embodiment. That is, for example, polyethylene, which is the base resin in the resin composition constituting the insulating layer, becomes crosslinked polyethylene.

[0141] In addition, the resin composition may also contain uncrosslinked polyethylene.

[0142] In this embodiment, when the inorganic filler is surface-treated with a vinyl silane coupling agent, at least a portion of the organic substituents of the silyl groups bonded to the surface of the inorganic filler are cross-linked (bonded) to, for example, the base resin. It should be noted that vinyl groups may remain in another portion of the silyl groups bonded to the surface of the inorganic filler.

[0143] (Outer semiconducting layer)

[0144] External semiconductive layer 140 is provided to cover the outer periphery of insulating layer 130. Furthermore, external semiconductive layer 140 is configured to have semiconductivity and to suppress electric field concentration between insulating layer 130 and shielding layer 150. External semiconductive layer 140 is formed of, for example, the same material as internal semiconductive layer 120.

[0145] (Shielding layer)

[0146] Shielding layer 150 is provided to cover the outer periphery of outer semiconductive layer 140. Shielding layer 150 is formed, for example, by winding copper tape or as a wire shield formed by winding multiple soft copper wires. It should be noted that tape, such as adhesive tape, may be wound around the inside and outside of shielding layer 150.

[0147] (jacket)

[0148] The sheath 160 is provided to cover the outer periphery of the shield layer 150. The sheath 160 is made of, for example, polyvinyl chloride or polyethylene.

[0149] (DC Characteristics)

[0150] In the DC power cable 10 configured as described above, the inorganic filler added to the insulating layer 130 has high adhesion to the base resin, thereby obtaining, for example, the following DC characteristics.

[0151] The following characteristics are obtained as DC characteristics of the DC power cable 10 in the initial state before a temperature change occurs.

[0152] In this embodiment, the volume resistivity of the sheet of the insulating layer 130 measured under the conditions of a temperature of 90° C. and a DC electric field of 80 kV / mm is, for example, 1×1015 Ω·cm or more, preferably 2×10 15 The thickness of the insulating layer 130 sheet is 0.15 mm.

[0153] In this embodiment, the dielectric breakdown field strength of the insulating layer 130 sheet measured at a temperature of 90° C. is, for example, 250 kV / mm or more, preferably 300 kV / mm or more. The thickness of the insulating layer 130 sheet is 0.15 mm.

[0154] In this embodiment, when a DC electric field of 50 kV / mm is applied to the sheet of insulating layer 130 at a temperature of 30° C. and atmospheric pressure, the field enhancement factor FEF (Field Enhancement Factor) obtained from the following formula (4) is, for example, less than 1.15.

[0155] FEF=E1 / (V0 / T)……(4)

[0156] Here, V0 is the voltage applied to the sheet of insulating layer 130 in kV, T is the thickness of the sheet of insulating layer 130 , 0.15 mm, and E1 is the maximum electric field inside the sheet of insulating layer 130 in kV / mm.

[0157] Furthermore, the following characteristics are obtained as DC characteristics of the DC power cable 10 after a predetermined heat cycle.

[0158] In the thermal cycle test, a cycle consisting of maintaining the DC power cable 10 at -10°C for 8 hours and maintaining the DC power cable 10 at room temperature (24°C) for 16 hours was repeated for three months. In this embodiment, after this thermal cycle test, a DC electric field of 200 kV / mm was applied through the thickness direction of the insulation layer 130 at a temperature of 90°C. The time until insulation breakdown of the insulation layer 130 was, for example, at least one hour, preferably at least two hours.

[0159] (Specific dimensions, etc.)

[0160] The specific dimensions of the DC power cable 10 are not particularly limited. For example, the diameter of the conductor 110 is 5 mm to 60 mm, the thickness of the inner semiconductive layer 120 is 1 mm to 3 mm, the thickness of the insulating layer 130 is 1 mm to 35 mm, the thickness of the outer semiconductive layer 140 is 1 mm to 3 mm, the thickness of the shielding layer 150 is 1 mm to 5 mm, and the thickness of the sheath 160 is 1 mm or more. The DC voltage applicable to the DC power cable 10 of this embodiment is, for example, 20 kV or more.

[0161] (3) Manufacturing method of DC power cable

[0162] Next, a method for manufacturing a DC power cable according to this embodiment will be described. Hereinafter, the step will be simply referred to as "S".

[0163] (S100: Resin Composition Preparation Step)

[0164] First, a resin composition comprising a base resin comprising polyolefin, a modified polyolefin having polar groups grafted thereto, and an inorganic filler comprising magnesium oxide is prepared. The resin composition preparation step S100 includes, for example, a surface treatment step S120 and a mixing step S140.

[0165] (S120: Surface treatment process)

[0166] First, at least a portion of the inorganic filler is surface-treated with a silane coupling agent. In this embodiment, the surface treatment step S120 is performed, for example, by a dry method.

[0167] First, a solution of a silane coupling agent diluted with an organic solvent is prepared. After the solution is prepared, an inorganic filler is added to a stirring device. The stirring device is, for example, a Henschel mixer.

[0168] After the inorganic filler is placed in a stirring device, a solution containing a silane coupling agent is added dropwise or sprayed using a sprayer while the inorganic filler is stirred in the stirring device, thereby surface-treating at least a portion of the inorganic filler with the silane coupling agent.

[0169] In this embodiment, in the surface treatment step S120, for example, the inorganic filler is surface treated to satisfy the necessary conditions for the inorganic filler's adhesion. Specifically, the inorganic filler is surface treated so that the residual ratio R of the inorganic filler in the freeze-fractured surface of the DC power cable 10 after manufacture is 50% or more, or so that the lightness V of the bent cross-section of the insulating layer 130 after manufacture of the DC power cable 10 satisfies equation (2).

[0170] Specifically, in the surface treatment step S120 of the present embodiment, the moisture content (absolute humidity, mass of water per unit volume) in the atmosphere during the surface treatment of the inorganic filler is set to, for example, 0.1 g / m 3 When the moisture content in the atmosphere exceeds 0.1g / m 3 When the amount of water in the atmosphere is 0.1 g / m3, the silane coupling agents may bond with each other. Therefore, the coverage of the inorganic filler by the silane coupling agent may decrease. In contrast, in this embodiment, by making the amount of water in the atmosphere 0.1 g / m3, the amount of water in the atmosphere 3This can suppress the bonding of silane coupling agents in the atmosphere, thereby suppressing a decrease in the coverage of the inorganic filler by the silane coupling agent.

[0171] It should be noted that the lower limit of the amount of water in the atmosphere is not particularly limited. However, in the surface treatment step S120, it is required to hydrolyze the hydrolyzable groups (alkoxy groups, etc.) in the silane coupling agent so that the hydrolyzed silane coupling agent is bonded to the surface of the polar inorganic filler. Therefore, from the perspective of hydrolyzing a predetermined amount of silane coupling agent, the water content in the atmosphere is preferably, for example, 0.0001 g / m 3 above.

[0172] To adjust the moisture content in the atmosphere to within the above range, the following methods can be used, for example. For example, the moisture content in the stirring device can be adjusted by supplying dry air with reduced moisture content into the stirring device. Alternatively, the moisture content in the stirring device can be adjusted by connecting a dehydration tower to the stirring device and circulating the atmosphere in the stirring device through the dehydration tower.

[0173] Furthermore, in the surface treatment step S120 of this embodiment, for example, an inorganic filler having an (initial) moisture content of 3 wt% or less is used. If the moisture content of the inorganic filler exceeds 3 wt%, the inorganic fillers may aggregate during the surface treatment step S120. In contrast, in this embodiment, by limiting the moisture content of the inorganic filler to 3 wt% or less, aggregation of the inorganic filler during the surface treatment step S120 can be suppressed. For this reason, a decrease in the coverage of the inorganic filler by the silane coupling agent can also be suppressed.

[0174] In addition, the lower limit of the water content of the inorganic filler is not particularly limited, but is, for example, 0.001 wt % from the viewpoint of stably producing the inorganic filler.

[0175] Here, the inorganic filler having a water content of 3 wt % or less can be obtained, for example, as follows.

[0176] For example, regardless of whether the method for forming the inorganic filler is a gas phase method or a seawater method, a calcination process (sintering process) is performed as the final step. Immediately after the calcination process, the moisture content of the inorganic filler becomes less than 3wt%. In this embodiment, after the calcination process of the inorganic filler, the inorganic filler is sealed and stored in an inert gas atmosphere, thereby maintaining the inorganic filler to be non-hygroscopic. Thus, the moisture content of the inorganic filler used in the surface treatment process S120 can be less than 3wt%.

[0177] On the other hand, when the water content of the prepared inorganic filler increases after a predetermined time has passed since the calcination process of the inorganic filler, the inorganic filler may be pre-dried before the surface treatment process S120 so that the water content becomes less than 3 wt%. As a method of pre-drying, for example, a method of heating the inorganic filler in an inert gas atmosphere may be cited. As a method of heating the inorganic filler, oven heating or microwave heating may be cited. When heating the inorganic filler, a cyclic purge process including a purge process using an inert gas and a vacuum process may be performed.

[0178] Furthermore, in the surface treatment step S120 of this embodiment, a solution obtained by diluting a silane coupling agent with an organic solvent having a water concentration of 7 vol% or less is used. If the water concentration of the organic solvent used for dilution exceeds 7 vol%, the silane coupling agents in the solution may bond with each other during dilution with the organic solvent. In contrast, in this embodiment, by setting the water concentration of the organic solvent used for dilution to 7 vol% or less, bonding of the silane coupling agents in the solution during dilution with the organic solvent can be suppressed. This prevents a decrease in the coverage of the inorganic filler by the silane coupling agent during surface treatment step S120.

[0179] In addition, the lower limit of the water concentration of the organic solvent is not particularly limited, but is, for example, 0.001 vol % from the viewpoint of stably purifying the organic solvent.

[0180] The organic solvent used to dilute the silane coupling agent is not particularly limited, and examples thereof include methanol, ethanol, isopropyl alcohol, and n-propyl alcohol. Two or more of these may be used in combination.

[0181] Other conditions in the surface treatment step S120 of the present embodiment are not particularly limited, and are set, for example, as follows.

[0182] The treatment temperature in the surface treatment step S120 of this embodiment is set to, for example, room temperature (ie, non-heating). Specifically, the treatment temperature in the surface treatment step S120 is set to, for example, 15° C. or higher and 25° C. or lower.

[0183] After the surface treatment step S120 is completed, the treated inorganic filler is appropriately dried. The drying method is not particularly limited, and examples thereof include vacuum drying.

[0184] Furthermore, the volume average particle size of the inorganic filler can be adjusted by performing a predetermined pulverization process. The final volume average particle size of the inorganic filler is set to, for example, 1 μm or less, preferably 700 nm or less.

[0185] (S140: Mixing Step)

[0186] A base resin comprising polyethylene, a modified polyolefin having polar groups grafted onto the polyolefin, an inorganic filler comprising magnesium oxide, a cross-linking agent comprising an organic peroxide, and other additives (antioxidants, lubricants, etc.) are mixed (kneaded) using a mixer such as a Banbury mixer or a kneader to form a mixed material. After the mixed material is formed, the mixed material is granulated using an extruder. This forms a granular resin composition that will constitute the insulating layer 130. It should be noted that a twin-screw extruder with a high mixing action can also be used to perform the steps from mixing to granulation at the same time.

[0187] (S200: Conductor Preparation Process)

[0188] On the other hand, a conductor 110 formed by twisting a plurality of conductor core wires is prepared.

[0189] (S300: Cable core forming process (extrusion process))

[0190] Next, in a three-layer simultaneous extruder, a resin composition for the inner semiconductive layer, for example, prepared by pre-mixing an ethylene-ethyl acrylate copolymer and conductive carbon black, is fed into the extruder A for forming the inner semiconductive layer 120 .

[0191] The pelletized resin composition is fed into the extruder B for forming the insulating layer 130 .

[0192] The outer semiconductive layer resin composition composed of the same material as the inner semiconductive layer resin composition fed into the extruder A is fed into the extruder C for forming the outer semiconductive layer 140 .

[0193] Next, the extrudates from extruders A through C are directed to a common head, where inner semiconductive layer 120, insulating layer 130, and outer semiconductive layer 140 are simultaneously extruded from the inside outward onto the outer circumference of conductor 110. Subsequently, insulating layer 130 is crosslinked by heating within a crosslinking tube pressurized with nitrogen or other gases, either through radiation from an infrared heater or heat transfer via a heat medium such as high-temperature nitrogen or silicone oil. This forms a cable core comprising conductor 110, inner semiconductive layer 120, insulating layer 130, and outer semiconductive layer 140.

[0194] (S400: Shielding Layer Forming Step)

[0195] Next, the shielding layer 150 is formed by winding, for example, a copper tape around the outer side of the outer semiconducting layer 140 .

[0196] (S500: Sheath Forming Step)

[0197] Next, vinyl chloride is fed into an extruder and extruded, thereby forming the sheath 160 on the outer periphery of the shield layer 150 .

[0198] In the above manner, the DC power cable 10 as a solid-insulated DC power cable is manufactured.

[0199] (4) Effects of this embodiment

[0200] According to this embodiment, one or more of the following effects are achieved.

[0201] (a) In this embodiment, by manufacturing the DC power cable 10 using the above-described manufacturing method, the adhesion of the inorganic filler added to the insulating layer 130 to the base resin can be stably improved and maintained over time. Specifically, the residual ratio R of the inorganic filler in the freeze-fracture surface can be maintained at 50% or greater. In other words, in this embodiment, even when the temperature is lowered to the extreme low temperature of liquid nitrogen, the inorganic filler can be maintained in a state of strong adhesion to the base resin.

[0202] Alternatively, in this embodiment, by improving the adhesion between the inorganic filler and the base resin, whitening at the bent cross-section of the insulating layer 130 can be suppressed, and the brightness V of the bent cross-section of the insulating layer 130 can be made less than (2.5N + 67.5)%. That is, in this embodiment, even if bending stress such as bending the insulating layer 130 completely 180 degrees is applied, the inorganic filler can be suppressed from peeling off from the base resin, and the formation of voids between the base resin and the inorganic filler can be suppressed.

[0203] By improving the adhesion between the inorganic filler and the base resin and maintaining this adhesion over time, even if the temperature in the installation environment of the DC power cable 10 fluctuates significantly, causing thermal expansion and contraction of the insulating layer 130, the inorganic filler can be prevented from peeling off from the base resin due to the thermal stress generated by the thermal expansion and contraction of the insulating layer 130. Furthermore, even if the temperature fluctuations are prolonged, causing the effects of thermal stress to accumulate in the insulating layer 130, the inorganic filler can be prevented from peeling off from the base resin due to the accumulation of thermal stress. By suppressing the peeling of the inorganic filler in this way, the formation of gaps between the base resin and the inorganic filler can be suppressed. This prevents localized degradation of the insulating properties of the insulating layer and reduces the inorganic filler's ability to capture space charge. As a result, degradation of the DC characteristics of the DC power cable 10 caused by temperature fluctuations can be suppressed.

[0204] (b) By ensuring that the residual ratio R of the inorganic filler in the frozen fracture surface is 50% or greater, or that the lightness V of the bent cross-section of the insulating layer 130 satisfies equation (2), the inorganic filler maintains high adhesion to the base resin even before a temperature change occurs. This allows the inorganic filler to fully capture space charge generated in the insulating layer 130, suppressing the accumulation of localized space charge in the insulating layer 130. Consequently, the DC characteristics of the DC power cable 10 in its initial state, before a temperature change occurs, can be improved.

[0205] (c) In the surface treatment step S120, the inorganic filler is surface treated so that the residual ratio R of the inorganic filler in the freeze-fracture surface is 50% or greater, or the lightness V of the bent cross-section of the insulating layer 130 satisfies equation (2). This maintains the adhesion between the inorganic filler and the base resin even when the temperature in the DC power cable installation environment fluctuates widely or over a long period of time. Consequently, degradation of the DC characteristics of the DC power cable due to temperature fluctuations can be suppressed.

[0206] (d) In the surface treatment step S120, the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.1 g / m 3 The following can suppress the bonding of silane coupling agents in the atmosphere. By suppressing the bonding of silane coupling agents in the atmosphere, the density of the silane coupling agents in the atmosphere can be suppressed from decreasing, and the coverage of the inorganic filler by the silane coupling agents can be suppressed from decreasing.

[0207] Furthermore, by setting the moisture content in the atmosphere during the surface treatment of the inorganic filler to 0.1 g / m 3 This can suppress moisture absorption by the inorganic filler. By suppressing moisture absorption by the inorganic filler, it is possible to suppress aggregation of the inorganic fillers during the surface treatment step S120. By suppressing aggregation of the inorganic fillers, the entire surface of each inorganic filler (each particle) can be exposed, allowing the entire surface of the inorganic filler to be surface treated with a silane coupling agent. For this reason, it is also possible to suppress a decrease in the coverage of the inorganic filler by the silane coupling agent.

[0208] As a result, it is possible to suppress a decrease in the adhesion between the inorganic filler and the base resin.

[0209] (e) In the surface treatment step S120, by using an inorganic filler with a water content of 3 wt% or less, the inorganic fillers can be prevented from agglomerating with each other. In particular, when the surface treatment step S120 is a dry process as in the present embodiment, the inorganic filler particles can be fully stirred, making it more difficult for the inorganic fillers to agglomerate with each other. This is similar to setting the moisture content in the atmosphere to 0.1 g / m 3The following effects are similarly achieved, and a decrease in the coverage of the inorganic filler by the silane coupling agent can be suppressed.

[0210] (f) The inorganic filler is preferably surface-treated with a silane coupling agent other than a monoalkylsilane coupling agent. While the detailed mechanism is unknown, this significantly improves the adhesion between the inorganic filler and the base resin compared to when the inorganic filler is surface-treated with a monoalkylsilane coupling agent. As a result, degradation of the DC characteristics of the DC power cable 10 due to temperature fluctuations can be significantly suppressed.

[0211] (g) Alternatively, the inorganic filler is preferably surface-treated with a silane coupling agent having a vinyl group at its terminal, so that the vinyl group derived from the vinyl silane coupling agent can be cross-linked with the base resin using a predetermined cross-linking agent.

[0212] Furthermore, as described above, in the surface treatment step S120, the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.1 g / m 3 This can increase the coverage of the inorganic filler by the vinyl silane coupling agent. This increases the number of crosslinking points between the silyl groups bonded to the surface of the inorganic filler and the base resin when the vinyl groups derived from the vinyl silane coupling agent are crosslinked with the base resin.

[0213] Thus, the synergistic effect of the vinyl silane coupling agent and the low-moisture surface treatment can significantly improve the adhesion between the inorganic filler and the base resin, thereby significantly suppressing degradation of the DC characteristics of the DC power cable 10 due to temperature changes.

[0214] <Other embodiments of the present disclosure>

[0215] As mentioned above, although embodiment of this disclosure was specifically described, this disclosure is not limited to the said embodiment, Various changes are possible within the range which does not deviate from the summary.

[0216] In the above embodiment, the resin composition includes a base resin containing a polyolefin and a modified polyolefin. However, the resin composition may also include a copolymer of an olefin and a polar monomer. Examples of copolymers of an olefin and a polar monomer include ethylene-ethyl acrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-butyl acrylate copolymers, ethylene-methyl methacrylate copolymers, and ethylene-glycidyl methacrylate copolymers. It should be noted that two or more of these copolymers may be used in combination.

[0217] In the above embodiment, the requirement that the inorganic filler has adhesiveness is satisfied is described assuming that the resin composition constituting the insulating layer 130 is cross-linked. However, this requirement may also apply to a case where the resin composition constituting the insulating layer 130 is not cross-linked. However, this requirement is more reliably satisfied when the resin composition constituting the insulating layer 130 is cross-linked.

[0218] Example

[0219] Next, embodiments of the present disclosure will be described. These embodiments are examples of the present disclosure, and the present disclosure is not limited to these embodiments.

[0220] (1) Experiment 1

[0221] (1-1) Samples of DC power cables

[0222] (1-1-1) Surface Treatment of Inorganic Fillers

[0223] As an inorganic filler, the following MgO was prepared. It should be noted that the inorganic filler was pre-dried so that the initial moisture content of the inorganic filler was 0.5wt%. In addition, the following silane coupling agent was diluted using ethanol with a moisture content of 1.5vol% as an organic solvent for dilution, thereby preparing a solution for surface treatment. Then, while adjusting the moisture content in the atmosphere in the stirring device to the following value, the inorganic filler was added to the stirring device and stirred. While stirring the inorganic filler in the stirring device, the above-mentioned surface treatment solution was added to the stirring device. In this way, the inorganic filler was surface treated by a dry method. It should be noted that the treatment temperature was set to room temperature (24°C). The detailed conditions are as follows.

[0224] Inorganic filler: Fumed MgO (volume average particle size X: 50 nm)

[0225] Silane coupling agents: vinyltrimethoxysilane (VTMS), vinyltriethoxysilane (VTE S), methyltrimethoxysilane (C1), octyltriethoxysilane (C8), octadecyltriethoxysilane (C18)

[0226] Dilution organic solvent: ethanol (water concentration 1.5 vol%)

[0227] Surface treatment method: dry method

[0228] (Initial) moisture content of inorganic filler: 0.5 wt%

[0229] Moisture content in the atmosphere: 0.01 to 10 g / m 3

[0230] (1-1-2) Production of Resin Composition

[0231] The following compounding ingredients were mixed using a Banbury mixer and pelletized using an extruder to produce a pelletized resin composition.

[0232] (Base resin)

[0233] Low density polyethylene (LDPE) (density d = 0.920 g / cm 3 , MFR=1g / 10min)

[0234] 100 parts by mass

[0235] (Modified polyolefin)

[0236] 0.5-15 parts by mass of maleic anhydride-modified polyethylene (MAH-PE, modification rate 0.5%)

[0237] (Inorganic filler)

[0238] MgO content N after the above-mentioned surface treatment: 1 to 2 parts by mass

[0239] (cross-linking agent)

[0240] 1.3 parts by mass of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane

[0241] (Other additives)

[0242] Lubricant: Specified amount of oleamide

[0243] Antioxidant: 4,4′-thiobis(3-methyl-6-tert-butylphenol) specified amount

[0244] (1-1-3) Manufacturing of DC Power Cable Samples

[0245] Next, a conductor formed by stranding conductor cores made of a low-concentration copper alloy (corresponding to "dilute copper alloy" in Japanese) with a diameter of 14 mm was prepared. After preparing the conductor, a resin composition for the internal semiconductive layer containing an ethylene-ethyl acrylate copolymer, the resin composition for the insulating layer prepared in the above (1-1-2), and a resin composition for the external semiconductive layer made of the same material as the resin composition for the internal semiconductive layer were respectively fed into extruders A to C. The respective extrudates from extruders A to C were guided to a common head, and the internal semiconductive layer, insulating layer, and external semiconductive layer were simultaneously extruded onto the outer periphery of the conductor from the inside to the outside. At this time, the thicknesses of the internal semiconductive layer, insulating layer, and external semiconductive layer were set to 1 mm, 3 mm, and 1 mm, respectively. Thereafter, the above extrusion-molded product was heated at about 250°C to crosslink the resin composition for the insulating layer. As a result, a sample of a DC power cable having a conductor, an internal semiconductive layer, an insulating layer, and an external semiconductive layer from the center to the outer periphery was manufactured.

[0246] Through the above processes, specimens A1-1 to A1-14, specimens B1-1 and B1-2 of DC power cables with different resin compositions were manufactured.

[0247] (1-₂) Evaluation

[0248] The following evaluations were respectively carried out on specimens A1-1 to A1-14, specimens B1-1 and B1-2 of the DC power cable.

[0249] (1-₂-₁) Evaluation of residual rate

[0250] [Sample processing]

[0251] By turning (corresponding to "cinnamon peeling" in Japanese) specimens A1-1 to A1-14, specimens B1-Ⅰ and B1-2 of the DC power cable from the outer peripheral surface respectively, sheets of the insulating layer with a thickness of 1 mm were formed from the central part in the thickness direction of the insulating layer. At this time, multiple sheets were formed from the same circumference.

[0252] (i) Density before freeze fracture

[0253] [Cutting surface processing]

[0254] The sheet of the insulating layer was cut using a focused ion beam at 24°C without freezing the sheet. At this time, gallium ions were used, the acceleration voltage was set to 30 kV, and the current value was set to 60 nA.

[0255] [Density evaluation of inorganic filler]

[0256] The cut surface of the sheet processed as described above was observed using a scanning electron microscope (SEM). The density of the inorganic filler in the cut surface was measured. It should be noted that the field of view of the cut surface was set to 15 μm x 10 μm, and the density of the inorganic filler in the cut surface was measured at 10 random locations, and the values were averaged. The density thus obtained was referred to as the "density Dc of the inorganic filler in the cut surface" described above.

[0257] (ii) Density after freeze fracture

[0258] [Formation of frozen fracture surface]

[0259] The insulating layer sheet was immersed in liquid nitrogen for 1 hour to freeze it, and then the sheet was broken by bending it.

[0260] [Density Evaluation of Inorganic Fillers]

[0261] The fracture surface (frozen fracture surface) of the sheet after fracture as described above was observed using an SEM. At this time, the density of the inorganic filler remaining in the frozen fracture surface was measured under the same conditions as those for the density of the cut surface described above. That is, the field of view of the frozen fracture surface was set to 15 μm × 10 μm, and the density of the residual inorganic filler was measured at 10 random frozen fracture surfaces, and these values were averaged. The density thus obtained was defined as the "density Df of the inorganic filler in the frozen fracture surface" described above.

[0262] (iii) Evaluation of residual rate

[0263] The residual ratio R of the inorganic filler in the frozen fracture surface was determined by substituting Dc and Df obtained in (i) and (ii) into the above-mentioned formula (1). As a result, a residual ratio R of less than 50% of the inorganic filler in the frozen fracture surface was evaluated as "poor," a residual ratio R of 50% or greater was evaluated as "good," and a residual ratio R of 60% or greater was evaluated as "optimal." It should be noted that in the following experiments, the conditions necessary for the residual ratio R to be good or optimal are also referred to as "required conditions for the residual ratio of the inorganic filler."

[0264] (1-2-2) Evaluation of Whitening of Insulation Layer

[0265] [Sample processing]

[0266] DC power cable samples A1-1 to A1-14, B1-1, and B1-2 were each turned from the outer peripheral surface to form an insulating layer sheet having a thickness of 2 mm, a width of 200 mm, and a length of 30 mm from the center in the thickness direction of the insulating layer.

[0267] [Measurement of Lightness V of Bent Cross Section of Sheet]

[0268] The insulating layer sheet was bent 180° at 24°C, and the cross-section of the bent portion was observed (photographed) using an optical microscope. Histogram analysis of the obtained cross-sectional image was performed using image analysis software to determine the value (V) in the HSV color space. The unit of value (V) is %. Furthermore, the center of the bent cross-section of the sheet was observed.

[0269] The observation conditions at this time were set to satisfy the following two requirements. The observation conditions were set so that the lightness V was 50% when observing a cross-section of an insulating layer sheet containing no MAH-PE and no inorganic filler (otherwise identical to the comparative sample in formulation) and bent under the same conditions as the comparative sample. Furthermore, the observation conditions were set so that the lightness V was 80% when observing a cross-section of an insulating layer sheet containing no MAH-PE and 5 parts by mass of an unsurface-treated inorganic filler (otherwise identical to the comparative sample in formulation) and bent under the same conditions as the comparative sample.

[0270] In this case, the visual field of the cross section formed by bending the sheet was set to 10 mm×5 mm, and the lightness V of 10 arbitrary cross sections was measured, and the values were averaged.

[0271] As a result, the case where V < 2.5N + 67.5 (i.e., satisfying equation (2)) was evaluated as "good", and the case where V < 5N + 55 (i.e., satisfying equation (3)) was evaluated as "optimal". It should be noted that this requirement in each experiment is also referred to as the "bend cross-section brightness requirement".

[0272] (1-2-3) Evaluation of initial DC characteristics

[0273] [Sample processing]

[0274] Samples A1-1 to A1-14, and samples B1-1 and B1-2 of the DC power cables were each turned from the outer peripheral surface to form a sheet having an insulating layer with a thickness of 0.15 mm.

[0275] [Volume resistivity]

[0276] The insulating layer sheet was immersed in silicone oil at a temperature of 90°C and a DC electric field of 80 kV / mm was applied to the insulating layer sheet using a flat electrode with a diameter of 25 mm to measure the volume resistivity. 15 The case of Ω·cm or more was evaluated as good.

[0277] [DC breakdown electric field strength]

[0278] The insulating layer sheet was immersed in silicone oil at 90°C. Using a 25 mm diameter flat electrode, the applied voltage was increased at a rate of 4 kV / min. The DC breakdown electric field strength of the insulating layer sheet was calculated by dividing the applied voltage at the time of dielectric breakdown by the sheet thickness. A DC breakdown electric field strength of 250 kV / mm or greater was considered good.

[0279] [Space Charge Characteristics]

[0280] The space charge characteristics of the insulating layer sheet were evaluated using a space charge measurement device (manufactured by Five Lab) based on the pulsed electrostatic stress method (PEA). Specifically, a 50 kV / mm DC electric field was continuously applied to the insulating layer sheet for one hour at 30°C and atmospheric pressure, and the maximum electric field within the sheet was measured. The electric field enhancement factor (FEF) was calculated using the above equation (4). An FEF less than 1.15 was rated A (good), while an FEF greater than 1.15 was rated B (poor).

[0281] (1-2-4) Evaluation of DC Characteristics after Thermal Cycling

[0282] [Thermal cycle test]

[0283] DC power cable samples A1-1 to A1-14, and samples B1-1 and B1-2 were placed in a low-temperature constant temperature bath and subjected to a thermal cycle test. The thermal cycle test was repeated for three months, with a step of maintaining the DC power cable at a temperature of -10°C for 8 hours and a step of maintaining the DC power cable at room temperature (24°C) for 16 hours.

[0284] [DC breakdown test]

[0285] After the thermal cycle test, the DC power cable was immersed in silicone oil at 90°C, and a DC electric field of 200 kV / mm was applied through the thickness of the insulation layer. A rating of A (optimal) was assigned for a time until insulation breakdown (hereinafter referred to as "insulation breakdown time") of 2 hours or longer, B (good) for a time of 1 hour or longer but less than 2 hours, and C (poor) for a time of less than 1 hour.

[0286] (1-3) Results

[0287] The results of the evaluation of the DC power cable samples in Experiment 1 are shown in the following Tables 1 and 2. In addition, in Tables 1 and 2, the unit of the content of the compounding agent is "parts by mass".

[0288] [Table 1]

[0289]

[0290] [Table 2]

[0291]

[0292] (Dependence on the amount of water during surface treatment)

[0293] [Samples B1-1 and B1-2]

[0294] During the surface treatment process, the moisture content in the atmosphere exceeds 0.1 g / m 3 In samples B1-1 and B1-2, the residual ratio R of inorganic filler in the freeze-fractured surface was less than 50%, meaning that the required residual ratio of inorganic filler was not met. Furthermore, in samples B1-1 and B1-2, V ≥ 2.5N + 67.5, meaning that the required bending cross-section brightness was not met.

[0295] In samples B1-1 and B1-2, the high moisture content in the atmosphere during the surface treatment process may have caused bonding between the silane coupling agents. Alternatively, the high moisture content in the atmosphere during the surface treatment process may have caused the inorganic fillers to absorb moisture, leading to aggregation of the inorganic fillers during this process. Therefore, it is believed that the coverage of the inorganic filler by the silane coupling agent decreased. As a result, it is believed that the adhesion between the inorganic filler and the base resin decreased in samples B1-1 and B1-2.

[0296] Sample B1-2 showed good initial DC characteristics (volume resistivity, DC breakdown electric field strength, and space charge characteristics), but Sample B1-1 showed poor results. In Sample B1-1, the adhesion between the inorganic filler and the base resin was too low, potentially leading to localized accumulation of space charge in the insulating layer. Consequently, Sample B1-1 exhibited poor initial DC characteristics.

[0297] The results of the DC breakdown test after the heat cycle showed that both samples B1-1 and B1-2 had poor results. In particular, it was found that sample B1-2 had good initial DC characteristics, but insulation breakdown occurred in a short period of time during the DC breakdown test after the heat cycle.

[0298] The above results indicate that the inorganic filler in samples B1-1 and B1-2 exhibited poor adhesion to the base resin and was unable to maintain this adhesion over time. Consequently, thermal stress during thermal cycling, or the accumulated effects of this thermal stress, caused the inorganic filler to peel from the base resin. This peeling of the inorganic filler may have locally reduced the insulation properties of the insulating layer sheet. As a result, it is believed that the insulation breakdown time in samples B1-1 and B1-2 after thermal cycling was shortened in the DC breakdown test.

[0299] [Samples A1-1 to A1-14]

[0300] During the surface treatment process, the moisture content in the atmosphere was set to 0.1 g / m 3 In the following samples A1-1 to A1-14, the residual ratio R of the inorganic filler in the freeze-fractured surface was 50% or greater, satisfying the inorganic filler residual ratio requirement. Furthermore, in samples A1-1 to A1-14, V < 2.5N + 67.5, satisfying the bent section brightness requirement.

[0301] In samples A1-1 to A1-14, by reducing the amount of moisture in the atmosphere during the surface treatment process, bonding between silane coupling agents in the atmosphere was suppressed. Alternatively, by reducing the amount of moisture in the atmosphere during the surface treatment process, moisture absorption by the inorganic filler was suppressed, thereby inhibiting aggregation of the inorganic fillers during this process. This suppressed a decrease in the coverage of the inorganic filler by the silane coupling agent. As a result, it was confirmed that in samples A1-1 to A1-14, a decrease in the adhesion between the inorganic filler and the base resin was suppressed.

[0302] Furthermore, samples A1-1 to A1-14 exhibited excellent initial DC characteristics (volume resistivity, DC breakdown electric field strength, and space charge characteristics). In samples A1-1 to A1-14, improved adhesion between the inorganic filler and the base resin suppressed localized space charge accumulation within the insulating layer. Consequently, it was confirmed that samples A1-1 to A1-14 exhibited excellent initial DC characteristics.

[0303] In addition, samples A1-1 to A1-14 showed good results in the DC breakdown test after thermal cycling. In samples A1-1 to A1-14, the adhesion between the inorganic filler and the base resin was improved and maintained over time. This prevented the inorganic filler from peeling off from the base resin due to thermal stress during thermal cycling or the accumulation of the effects of this thermal stress. By suppressing the peeling of the inorganic filler, the insulation properties of the insulating layer were prevented from being locally reduced. As a result, it was confirmed that samples A1-1 to A1-14 were able to extend the insulation breakdown time in the DC breakdown test after thermal cycling.

[0304] In addition, the moisture content in the atmosphere during the surface treatment process was set to 0.1 g / m 3 Samples A1-3, A1-8, and A1-9, which varied within the following ranges, all met the requirements for the residual rate of the inorganic filler and the required conditions for the brightness of the bent cross-section, and the results of the initial DC characteristics and the DC breakdown test after the thermal cycle were good. Therefore, it was confirmed that even if the moisture content in the atmosphere during the surface treatment step was 0.1 g / m 3 Even when the amount is changed within the following range, the adhesion-improving effect and the insulation-improving effect of the inorganic filler can be stably obtained.

[0305] (MAH-PE content dependence)

[0306] In samples A1-1 to A1-7, which varied the MAH-PE content, all met the requirements for the residual rate of the inorganic filler and the required conditions for the bending cross-section brightness, and the results of the initial DC characteristics and the DC breakdown test after the thermal cycle were good. Therefore, it was confirmed that by setting the moisture content in the atmosphere during the surface treatment step to 0.1 g / m 3 Hereinafter, regardless of the content of MAH-PE, the adhesion-improving effect and the insulation-improving effect of the inorganic filler can be stably obtained.

[0307] (Dependence on the content of inorganic filler)

[0308] Samples A1-3 and A1-10, in which the inorganic filler content N was varied, met the required inorganic filler residual ratio. Furthermore, samples A1-3 and A1-10 met the required bending cross-sectional brightness (V < 2.5N + 67.5) with the content N as a variable. Samples A1-3 and A1-10 showed good results in both the initial DC characteristics and the DC breakdown test after thermal cycling.

[0309] Although not shown in Tables 1 and 2, it was confirmed that the initial DC characteristics and the results of the DC breakdown test after heat cycling were also good in samples in which the content N of the inorganic filler was 0.1 parts by mass or 5 parts by mass and other conditions were the same as those of Sample A1-3.

[0310] From the above, it was confirmed that by setting the moisture content in the atmosphere during the surface treatment step to 0.1 g / m 3 Hereinafter, regardless of the content N of the inorganic filler, the adhesion-improving effect and the insulation-improving effect of the inorganic filler can be stably obtained.

[0311] (Dependence on organic substituents of silane coupling agents)

[0312] Samples A1-11 to A1-13, which used a monoalkylsilane coupling agent, met the inorganic filler residual ratio R of 50% or greater in the frozen fracture surface, a requirement for inorganic filler residual ratio. However, this residual ratio R was less than 60%. Furthermore, while samples A1-11 to A1-13 met the bending cross-sectional brightness requirement of V < 2.5N + 67.5, V ≥ 5N + 55. In DC breakdown tests after thermal cycling, samples A1-11 to A1-13 achieved better results than samples B1-1 and B1-2, but the dielectric breakdown time was less than 2 hours.

[0313] In contrast, samples A1-3 and A1-14, which used a vinyl silane coupling agent as a silane coupling agent other than a monoalkyl silane coupling agent, had an inorganic filler residual ratio (R) of 60% or greater in the freeze-fracture surface, and V < 5N + 55. In DC breakdown tests after thermal cycling, samples A1-3 and A1-14 each had a dielectric breakdown time of 2 hours or greater.

[0314] Based on the above, it was confirmed that the use of a silane coupling agent other than a monoalkylsilane coupling agent can significantly improve the adhesion between the inorganic filler and the base resin. In particular, in Samples A1-3 and A1-14, the synergistic effect of the vinyl silane coupling agent and the low-moisture surface treatment significantly improved the adhesion between the inorganic filler and the base resin. As a result, a significant effect of improving insulation properties was confirmed.

[0315] (Dependence of hydrolyzable groups of silane coupling agents)

[0316] In samples A1-3 and A1-14, which used silane coupling agents with different hydrolyzable groups, the residual rate R of inorganic filler in the freeze-fracture surface and the brightness V of the bent cross section were approximately the same. In samples A1-3 and A1-14, the initial DC characteristics and the DC breakdown test results after thermal cycling were equivalent. Based on the above, it was confirmed that by setting the moisture content in the atmosphere during the surface treatment process to 0.1 g / m 3 Hereinafter, regardless of the type of the hydrolyzable group of the silane coupling agent, the adhesion-improving effect and the insulation-improving effect of the inorganic filler can be stably obtained.

[0317] (2) Experiment 2

[0318] (2-1) Samples of DC power cables

[0319] DC power cable samples A2-1 to A2-14, B2-1, and B2-2 were produced in the same manner as samples A1-1 to A1-14, B1-1, and B1-2 in Experiment 1, except that seawater-processed MgO (volume average particle size X: 500 nm) was used as the inorganic filler.

[0320] (2-2) Evaluation

[0321] Evaluation was performed in the same manner as in Experiment 1 on samples A2-1 to A2-14 and samples B2-1 and B2-2 of the DC power cables.

[0322] (2-3) Results

[0323] The results of the evaluation of the DC power cable samples in Experiment 2 are shown in the following Tables 3 and 4. In addition, in Tables 3 and 4, the unit of the content of the compounding agent is "parts by mass".

[0324] [Table 3]

[0325]

[0326] [Table 4]

[0327]

[0328] (Dependence on the type of inorganic filler and the particle size of the inorganic filler)

[0329] In Experiment 2 using seawater-processed MgO having a volume average particle size X of 500 nm, the same tendency as in Experiment 1 was obtained regarding the moisture content in the atmosphere during the surface treatment step.

[0330] Specifically, the moisture content in the atmosphere is set to exceed 0.1 g / m 3 Samples B2-1 and B2-2 did not meet the requirements for the residual inorganic filler ratio and the bending cross-sectional brightness. Samples B2-1 and B2-2 exhibited poor initial DC characteristics and poor DC breakdown test results after thermal cycling.

[0331] In contrast, the moisture content in the atmosphere was set to 0.1 g / m 3 The following samples A2-1 to A2-14 satisfied the inorganic filler residual ratio requirement and the bent cross-section brightness requirement. It was confirmed that the initial DC characteristics and the DC breakdown test results after heat cycling were good in the samples A2-1 to A2-14.

[0332] The results of Experiments 1 and 2 confirmed that by setting the moisture content in the atmosphere during the surface treatment step to 0.1 g / m 3 Hereinafter, regardless of the method for forming MgO as the inorganic filler, the adhesion-improving effect and the insulation-improving effect of the inorganic filler can be stably obtained.

[0333] It should be noted that, although not recorded in Tables 1 to 4, it was confirmed that the initial DC characteristics and the results of the DC breakdown test after thermal cycling were also good in the sample in which the volume average particle size X of the gas-phase process MgO was 1 nm and the other conditions were the same as those of sample A1-3, and in the sample in which the volume average particle size X of the seawater process MgO was 1 μm and the other conditions were the same as those of sample A2-3.

[0334] According to the results of Experiment 1, Experiment 2 and the above-mentioned supplementary experiment, it was confirmed that by setting the moisture content in the atmosphere during the surface treatment step to 0.1 g / m 3 The following results show that even when the volume average particle size X of the inorganic filler is varied (assuming the content N remains constant), the required conditions for the residual ratio of the inorganic filler and the required conditions for the bending cross-sectional brightness are satisfied. It was confirmed that satisfying these requirements can achieve the effects of improving adhesion and insulation properties of the inorganic filler.

[0335] (Other dependencies in Experiment 2)

[0336] The comparison results of samples A2-3, A2-8, and A2-9 confirmed that when MgO different from that in Experiment 1 was used, even when the moisture content in the atmosphere during the surface treatment step was set to 0.1 g / m 3 Even when the amount is changed within the following range, the adhesion-improving effect and the insulation-improving effect of the inorganic filler can be stably obtained.

[0337] The comparison results of samples A2-1 to A2-7, samples A2-3 and A2-10, and samples A2-3 and A2-14 confirmed that even when MgO different from that in Experiment 1 was used, the surface treatment process could be performed with a water content of 0.1 g / m 3 Hereinafter, regardless of the content of MAH-PE, the content N of the inorganic filler, and the type of the hydrolyzable group of the silane coupling agent, the adhesion-enhancing effect and the insulation-enhancing effect of the inorganic filler can be stably obtained.

[0338] Comparison of samples A2-11 to A2-13 with samples A2-3 and A2-14 confirmed that even when MgO different from that in Experiment 1 was used, the use of a silane coupling agent other than a monoalkylsilane coupling agent significantly improved the adhesion between the inorganic filler and the base resin.

[0339] <Preferred embodiment of the present disclosure>

[0340] Preferred aspects of the present disclosure are described below.

[0341] (Note 1)

[0342] A resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted onto the polyolefin; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein a residual ratio R of the inorganic filler in a frozen fracture surface is 50% or more, wherein the residual ratio R is obtained by the following formula (1): R=(Df / Dc)×100……(1), Df is the density of the inorganic filler remaining in the fracture surface of a sheet of the resin composition comprising the base resin, the modified polyolefin, and the inorganic filler, and the sheet is immersed in liquid nitrogen for 1 hour and then fractured, and Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet of the resin composition is cut at 24°C using a focused ion beam without being frozen.

[0343] (Note 2)

[0344] A power cable comprises: a conductor; and an insulating layer arranged to cover the outer periphery of the conductor, the insulating layer being composed of a resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted thereto; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein a residual ratio R of the inorganic filler in a frozen fracture surface is 50% or more, wherein the residual ratio R is obtained by the following formula (1): R=(Df / Dc)×100……(1), where Df is the density of the inorganic filler remaining in the fracture surface of a sheet forming the insulating layer when the sheet is immersed in liquid nitrogen for 1 hour and then fractured, and Dc is the reference density of the inorganic filler detected in the fracture surface of the sheet when the sheet is cut at 24°C using a focused ion beam without being frozen.

[0345] (Note 3)

[0346] The power cable according to Note 2, wherein, after a prescribed thermal cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 1 hour, wherein, in the thermal cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0347] (Note 4)

[0348] The power cable according to Supplementary Note 2 or 3, wherein the inorganic filler is surface-treated with a material other than a silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon atom as the silane coupling agent.

[0349] (Note 5)

[0350] The power cable according to Supplementary Note 4, wherein the inorganic filler is selected from the group consisting of vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyl The surface of the present invention is treated with at least one of 1,3-dimethylbutylene)propylamine, 2,4-dimethoxy-2,4-dichloro-2,4-dimethyl-3-thiazolyl-1,2-dimethyl-1,4-dimethyl-2,4-dichloro-2,4-dimethyl-3-thiazolyl-1,2 ...

[0351] (Note 6)

[0352] The electric power cable according to any one of Supplementary Notes 2 to 5, wherein the inorganic filler is surface-treated with a material having a vinyl group at a terminal as the silane coupling agent.

[0353] (Note 7)

[0354] The electric power cable according to any one of Supplementary Notes 4 to 6, wherein the residual ratio R is 60% or more.

[0355] (Note 8)

[0356] The power cable according to any one of Notes 4 to 7, wherein, after a prescribed thermal cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 2 hours, wherein, in the thermal cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0357] (Note 9)

[0358] A power cable comprises: a conductor; and an insulating layer provided to cover the outer periphery of the conductor, wherein the insulating layer is composed of a resin composition, the resin composition comprising: a base resin comprising a polyolefin; a modified polyolefin having a polar group grafted thereto; and an inorganic filler comprising magnesium oxide and surface-treated with a silane coupling agent, wherein the content N of the inorganic filler in the resin composition is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin, and the following formula (2) is satisfied: V < 2.5N + 67.5 ... (2), Here, V is the lightness in the HSV color space, with the unit being % when a sheet having the insulating layer having a thickness of 2 mm is formed, the sheet is bent at 24°C, and the cross-section of the bent portion of the sheet is observed. When observing the cross-section of a sheet having an insulating layer not containing the modified polyolefin and the inorganic filler and bent under the same conditions, V is set to 50%. Furthermore, when observing the cross-section of a sheet having an insulating layer not containing the modified polyolefin and containing 5 parts by mass of an unsurface-treated inorganic filler and bent under the same conditions, V is set to 80%.

[0359] (Note 10)

[0360] The power cable according to Note 9, wherein, after a prescribed thermal cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 1 hour, wherein, in the thermal cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0361] (Note 11)

[0362] The power cable according to Supplementary Note 9 or 10, wherein the inorganic filler is surface-treated with a material other than a silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon atom as the silane coupling agent.

[0363] (Note 12)

[0364] The power cable according to Supplementary Note 11, wherein the inorganic filler is selected from the group consisting of vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane, and the like. The surface of the composite material is treated with at least one of oxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylene)propylamine, dimethyldimethoxysilane, methoxytrimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, and trimethylchlorosilane.

[0365] (Note 13)

[0366] The electric power cable according to any one of Supplementary Notes 9 to 12, wherein the inorganic filler is surface-treated with a material having a vinyl group at a terminal as the silane coupling agent.

[0367] (Note 14)

[0368] The power cable according to any one of Supplementary Notes 11 to 13, wherein the following formula (3) is satisfied: V<5N+55 (3).

[0369] (Note 15)

[0370] The power cable according to any one of Notes 11 to 14, wherein, after a prescribed thermal cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulation layer at a temperature of 90°C, the time until insulation breakdown occurs in the insulation layer is more than 2 hours, wherein, in the thermal cycle test, a cycle comprising the steps of maintaining the power cable at a temperature of -10°C for 8 hours and maintaining the power cable at 24°C for 16 hours is repeated for three months.

[0371] (Note 16)

[0372] A method for manufacturing a power cable comprises the following steps: preparing a resin composition comprising a base resin including a polyolefin, a modified polyolefin having a polar group grafted onto the polyolefin, and an inorganic filler including magnesium oxide; and forming an insulating layer using the resin composition to cover the outer periphery of a conductor, wherein the step of preparing the resin composition comprises a step of surface-treating the inorganic filler with a silane coupling agent, wherein the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.1 g / m 3 Hereinafter, the residual rate R of the inorganic filler in the frozen fracture surface is made to be greater than 50%, wherein the residual rate R is calculated by the following formula (1): R = (Df / Dc) × 100……(1), Df is the density of the inorganic filler remaining in the fracture surface of the sheet forming the insulating layer, when the sheet is immersed in liquid nitrogen for 1 hour and then the sheet is fractured, and Dc is the reference density of the inorganic filler detected in the cut surface of the sheet when the sheet is not frozen but cut using a focused ion beam at 24°C.

[0373] (Note 17)

[0374] A method for manufacturing a power cable comprises the following steps: preparing a resin composition comprising a base resin comprising a polyolefin, a modified polyolefin having a polar group grafted onto the polyolefin, and an inorganic filler comprising magnesium oxide, wherein the content N of the inorganic filler is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin; and forming an insulating layer by using the resin composition to cover the outer periphery of a conductor, wherein the step of preparing the resin composition comprises the step of surface-treating the inorganic filler with a silane coupling agent, wherein in the step of surface-treating the inorganic filler, the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.1 g / m 3Hereinafter, the following formula (2) is satisfied, V<2.5N+67.5……(2), wherein V is the brightness in the HSV color space with the unit being % when a sheet having the insulating layer having a thickness of 2 mm is formed, the sheet is bent at 24°C, and the cross section of the bent portion of the sheet is observed, V is set to 50% when a cross section is observed when a sheet having an insulating layer not containing the modified polyolefin and the inorganic filler is bent under the same conditions, and V is set to 80% when a sheet having an insulating layer not containing the modified polyolefin and the inorganic filler is bent under the same conditions.

[0375] (Note 18)

[0376] The method for producing a power cable according to Supplementary Note 16 or 17, wherein in the step of surface-treating the inorganic filler, the inorganic filler having a water content of 3 wt % or less is used.

[0377] (Note 19)

[0378] The method for producing a power cable according to any one of Supplementary Notes 16 to 18, wherein in the step of surface-treating the inorganic filler, a solution obtained by diluting the silane coupling agent with an organic solvent having a water concentration of 7 vol % or less is used.

[0379] Description of Reference Numerals

[0380] 10: DC power cable

[0381] 110: Conductor

[0382] 120: Inner semi-conductive layer

[0383] 130: Insulation layer

[0384] 140: Outer semi-conductive layer

[0385] 150: Shielding layer

[0386] 160: Sheath.

Claims

1. A resin composition comprising: a base resin comprising a polyolefin; Modified polyolefins having polar groups grafted thereto; and The inorganic filler contains magnesium oxide and is surface treated with a silane coupling agent. In the step of surface treating the inorganic filler, the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.0001 g / m 3 Above and 0.1g / m 3 Hereinafter, the residual ratio R of the inorganic filler in the frozen fracture surface is 50% or more, in, The residual rate R is obtained by the following formula (1): R=(Df / Dc)×100……(1), Df is the density of the inorganic filler remaining on the fracture surface of a sheet of a resin composition comprising the base resin, the modified polyolefin, and the inorganic filler, after the sheet is immersed in liquid nitrogen for 1 hour and then fractured. Dc is the reference density of the inorganic filler detected in the cut surface of the resin composition sheet when the sheet is cut using a focused ion beam at 24° C. without being frozen. The moisture content of the inorganic filler is 3% by weight or less, The content of the inorganic filler in the resin composition is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin.

2. A power cable comprising: conductors; and an insulating layer, provided to cover the outer periphery of the conductor, The insulating layer is composed of a resin composition comprising: a base resin including polyolefin; a modified polyolefin having polar groups grafted thereto; and an inorganic filler including magnesium oxide and surface-treated with a silane coupling agent. In the step of surface treating the inorganic filler, the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.0001 g / m 3 Above and 0.1g / m 3 Hereinafter, the residual ratio R of the inorganic filler in the frozen fracture surface is 50% or more, in, The residual rate R is obtained by the following formula (1): R=(Df / Dc)×100……(1), Df is the density of the inorganic filler remaining on the fracture surface of the sheet forming the insulating layer, when the sheet is immersed in liquid nitrogen for 1 hour and then fractured. Dc is the reference density of the inorganic filler detected in the cut surface of the insulating layer sheet when the sheet is cut using a focused ion beam at 24° C. without being frozen. The moisture content of the inorganic filler is 3% by weight or less, The content of the inorganic filler in the resin composition is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin.

3. The power cable according to claim 2, wherein: After a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulating layer at a temperature of 90°C, the time until the insulating layer undergoes dielectric breakdown is 1 hour or longer. In the heat cycle test, a cycle including a step of maintaining the power cable at a temperature of -10°C for 8 hours and a step of maintaining the power cable at 24°C for 16 hours was repeated for three months.

4. The power cable according to claim 2 or 3, wherein: The inorganic filler is surface-treated with a material other than the silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon atom.

5. The power cable according to claim 4, wherein: The inorganic filler is prepared by using, as the silane coupling agent, vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane. The surface of the present invention is treated with at least one of alkyl, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylene)propylamine, dimethyldimethoxysilane, methoxytrimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, and trimethylchlorosilane.

6. The power cable according to claim 2 or 3, wherein: The inorganic filler is surface-treated with a material having a vinyl group at a terminal, which is the silane coupling agent.

7. The power cable according to claim 4, wherein: The residual rate R is 60% or more.

8. The power cable according to claim 4, wherein: After a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulating layer at a temperature of 90°C, the time until the insulating layer undergoes dielectric breakdown is 2 hours or more. In the heat cycle test, a cycle including a step of maintaining the power cable at a temperature of -10°C for 8 hours and a step of maintaining the power cable at 24°C for 16 hours was repeated for three months.

9. A power cable comprising: conductors; and an insulating layer, provided to cover the outer periphery of the conductor, The insulating layer is composed of a resin composition comprising: a base resin including polyolefin; a modified polyolefin having polar groups grafted thereto; and an inorganic filler including magnesium oxide and surface-treated with a silane coupling agent. The content N of the inorganic filler in the resin composition is 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the base resin. Furthermore, in the step of surface treating the inorganic filler, the moisture content in the atmosphere during the surface treatment of the inorganic filler is set to 0.0001 g / m 3 Above and 0.1g / m 3 The following equation (2) is satisfied: V<2.5N+67.5……(2), in, V is the brightness in the HSV color space, with the unit being % when a sheet having the insulating layer having a thickness of 2 mm is formed, the sheet is bent at 24° C., and a cross section of the bent portion of the sheet is observed. V is set to 50% when observing a cross section of a sheet of an insulating layer not containing the modified polyolefin and the inorganic filler and bent under the same conditions, Furthermore, V is set to 80% when observing a cross section of a sheet containing no modified polyolefin and 5 parts by mass of an unsurface-treated inorganic filler and bent under the same conditions. The inorganic filler has a moisture content of 3 wt % or less.

10. The power cable according to claim 9, wherein: After a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulating layer at a temperature of 90°C, the time until the insulating layer undergoes dielectric breakdown is 1 hour or longer. In the heat cycle test, a cycle including a step of maintaining the power cable at a temperature of -10°C for 8 hours and a step of maintaining the power cable at 24°C for 16 hours was repeated for three months.

11. The power cable according to claim 9 or 10, wherein: The inorganic filler is surface-treated with a material other than the silane coupling agent having three hydrolyzable groups and only one alkyl group per silicon atom.

12. The power cable according to claim 11, wherein: The inorganic filler is prepared by using, as the silane coupling agent, vinyltrimethoxysilane, vinyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropyltriethoxysilane. The surface of the present invention is treated with at least one of alkyl, 3-acryloxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylene)propylamine, dimethyldimethoxysilane, methoxytrimethylsilane, hexamethyldisilazane, dimethyldichlorosilane, and trimethylchlorosilane.

13. The power cable according to claim 9 or 10, wherein: The inorganic filler is surface-treated with a material having a vinyl group at a terminal, which is the silane coupling agent.

14. The power cable according to claim 11, wherein Satisfying the following formula (3), V<5N+55……(3).

15. The power cable according to claim 11, wherein After a prescribed heat cycle test, when a DC electric field of 200 kV / mm is applied in the thickness direction of the insulating layer at a temperature of 90°C, the time until the insulating layer undergoes dielectric breakdown is 2 hours or more. In the heat cycle test, a cycle including a step of maintaining the power cable at a temperature of -10°C for 8 hours and a step of maintaining the power cable at 24°C for 16 hours was repeated for three months.

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